1. Field of the Invention
The present invention relates generally to restoration of film. More particularly, the invention concerns a method and apparatus for the restoration and chemical stabilizing of acetate base motion picture films.
2. Discussion of the Prior Art
Motion pictures have come to represent an increasingly important aspect of our cultural heritage, and like other art forms, their preservation offers serious challenges to archivists. One of the most severe problems is the deterioration of acetate films due to the condition generally referred to as “vinegar syndrome.”
First noticed in the 1950's in films stored at high humidity, “vinegar syndrome” is a progressively ruinous condition for which all acetate films are at risk. Acetate is an ester of cellulose which is produced with large quantities of acetic acid. An ester, by definition, is a substance that can be broken down by heating in the presence of a catalyst. This process is termed hydrolysis.
When acetate film begins to become hydrolysized, it undergoes a reductive change in its chemical make up, with dilute acetic acid forming on its surface, and a distinctive vinegar like smell emanating there from. This change begins slowly and proceeds exponentially with the increasing presence of acetic acid acting as an ever more powerful catalyst. Ultimately, the film is effectively destroyed.
Despite the problem of “vinegar syndrome”, acetate film is usually considered the most proven archival storage medium for motion pictures. Earlier cinema, starting around 1895, was produced on nitrate film, a highly flammable, chemically unstable, yet strong and reliable (when new) film base. Because of the danger inherent in the handling and storage of nitrate film, attempts were made early in the twentieth century to create films that were non-flammable. Acetate films (based on acetic acid rather than nitric acid) were developed with this objective in mind. Early acetate films possessed notably less tensile strength than their nitrate counterparts. For this and other reasons, nitrate film was continuously used for both the production and exhibition of theatrical motion pictures until approximately 1950. Up to about 1950 acetate films were used primarily for home movies and to stock rental libraries for schools, churches, and homes.
In the late 1940's the Eastman Kodak Company developed an improved acetate film stock known as triacetate. The more complex molecular bonds of this film provided greater tensile strength and allowed this stock to replace nitrate base film in the field of theatrical motion pictures. Most of the surviving significant films of the nitrate era have been preserved on acetate film. Acetate film remains in use today for the filming of theatrical motion pictures.
It was initially thought that the improved acetate film would provide a long term, trouble free solution to the instability problems posed by nitrate film, but this proved to be not entirely true. “Vinegar syndrome” was often found to affect prints that had been subjected to “scratch treatments”, that is, chemical processes designed to lighten or eliminate scratches on the film's emulsion or base. Then too, magnetic sound track films with their iron oxide coatings acting as a catalyst frequently became “vinegary”. Finally, the condition surfaced in random well stored negatives and prints with no unusual history.
Essentially, acetate films are the product of a series of reactions that, in chemical terms, do not go to completion. This means that they represent a chemical balance, rather than a stable compound, and that balance can be upset. As time passes and films age, their chance of developing this imbalance increases.
As instances of “vinegar syndrome” have become more pervasive, various suggestions have been made to slow the condition's progress. However, in dealing with the problems of preventing or curing “vinegar syndrome”, the prior art has failed to disclose or remotely suggest the novel processes of the present invention, and therefore, is generally irrelevant.
Archival practice is to store acetate films in low humidity conditions at low temperatures, but above freezing. This tends to slow, but not arrest, the progress of “vinegar syndrome”. Archival film storage cans, made of plastic instead of metal (metal is considered a catalyst for the syndrome) and designed to allow the escape of acid fumes, have been brought into common use. Additionally, silica gel packs are often used to remove water from film, as water is the fuel of hydrolysis. Probably the most effective, generally accepted tool specifically designed to combat “vinegar syndrome” is Eastman Kodak's Molecular Sieve. Molecular Sieves are similar to silica gel packets and are placed in film cans to absorb water and acetic acid. While effective in slowing “vinegar syndrome”, these products fail to return films to full stability.